Rotation and translation measurement

ABSTRACT

A position determining system (PDS)( 100 ) or multiple parameters measurement system (MPMS)( 300 ) particularly useful for translation and rotation measurement of objects having up to five degrees of freedom. A light source ( 122, 302 ) provides light beams ( 124, 304 ) into at least two channels. Each channel may include an interferometer ( 310 ), reflective target ( 110, 314 ), beam splitter ( 128, 312 ), detector ( 132, 316 ), and receiver ( 318 ). In concert, the detectors ( 132, 316 ) sense horizontal, vertical, and roll position and the receivers ( 318 ) sense longitudinal and yaw position change. Optionally, modulation can be imposed on the light beams ( 124, 304 ) and phase-sensitive synchronous demodulation used to enhance accuracy.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.09/434,100 filed Nov. 5, 1999, now U.S. Pat. No. 6,316,779 which is acontinuation-in-part of U.S. application Ser. No. 08/812,998 filed Mar.4, 1997, now U.S. Pat. No. 5,991,112 issued Nov. 23, 1999.

TECHNICAL FIELD

The present invention relates generally to the field of opticalmeasurement, and more particularly to accurately detecting positionalcharacteristics of a fixed or moving measurement target. It isanticipated that primary applications of the present invention will bein manufacturing of highly precise assemblies and in industrial andlaboratory processes requiring high precision position detection andcontrol.

BACKGROUND ART

Many present industries and fields of research are encountering a needfor faster and more accurate position and movement determination. Forexample, in semiconductor fabrication and disk drive assembly thecapacity of the ultimate end product depends highly on the accuracy ofthe measurement systems used, while the economy of the product oftendepends highly on the speed of the measurement systems used.

Many different measurement systems exist and are in wide use today. Ofpresent interest are optical measurement systems, since they oftenpermit non-contact measurement and have many other desirablecharacteristics. Present optical systems range from simple triangulationsystems which use light beam reflection and geometric principles knownsince ancient times, to complex laser systems which use interferometricprinciples to achieve accuracy to within fractions of one lightwavelength. However, particularly as modem applications becomeincreasingly complex, the often conflicting goals of measurementaccuracy and manufacturing speed remain ones where many seek furtherimprovement.

FIG. 1 (background art) stylistically depicts a measurement system 10for determining positional information about a movement stage 12. AsFIG. 1 illustrates with linear and circular arrowed lines, the movementstage 12 can have its position defined with respect to numerouscoordinate systems. For example, positional information about themovement stage 12 can be with respect to each of x, y, and z linearaxes, as well as with respect to each of rotational axes for pitch, yaw,and roll. The movement stage 12 thus can be viewed as having as many ashas six degrees of freedom. Of course, and as often is the case,movement may be limited to only some of or may not be of interest inonly some of these degrees of freedom, but accurate and fast measurementis still often a daunting task.

FIG. 1 includes a first detector 14, a second detector 16, a thirddetector 18, a controller 20, and an external system 22. The firstdetector 14 can detect position relative to the x-axis, and providepositional information with respect to this to the controller 20. Thesecond detector 16 can detect position or displacement relative to they-axis, and provide further positional information about this to thecontroller 20. The third detector 18 can detect position or displacementrelative to the z-axis and provide information about this to thecontroller 20.

Practitioners of the optical measurement arts will recall that manycommon detectors today are only able to detect positional change. Forexample, interferometers can only detect target displacement, a relativeposition measurement, and not initial or absolute position. Further, ifdisplacement occurs too slow or too fast even these techniques willfail. Herein we generally discuss absolute and relative measurementtechniques collectively unless particular differences are important.

Returning to FIG. 1, the controller 20 there provides the positionalinformation it receives, perhaps after appropriate processing and formatconversion, to the external system 22. The external system 22 may simplybe a display unit that a human user reads, or it may be a servo feedbacksystem precisely controlling various movements of the movement stage 12in a complex manufacturing process. The external system 22 is thus“external” with respect to the measurement process used; it is merely arecipient of and an ultimate user of the results of the measurementsystem for some higher purpose.

Unfortunately, the simple position determining system of FIG. 1 can onlyprovide positional information about three degrees of freedom for themovement stage 12. It cannot, for example, tell us anything about rollas depicted by the rotational arrowed line 24. Using detectors of thesort depicted here, adding roll detection would require adding at leasta fourth detector 26 (depicted in ghost form) in parallel with thesecond detector 16. Doing this would thus entail the expenses of moredetector hardware, increased controller capability to handle theadditional burden of this, and the attendant set-up and maintenance ofthe more complex position determining system which would result. If thedetectors which are used are laser interferometers, as might very wellbe the case today in a manufacturing or laboratory scenario where highaccuracy is necessary, the expense of another detector could be quiteappreciable. The costs of additional set-up and maintenance would alsolikely be appreciable. However, and worth noting for later in thisdiscussion, the added cost for increased controller capability might bequite negligible.

Accordingly, what is needed is a position determining system whichemploys relatively simple detection hardware yet provides positionalinformation for a measurement target with respect to multiple axes.

DISCLOSURE OF INVENTION

Accordingly, it is an object of the present invention to provide aposition determining system which provides information about ameasurement target with respect to multiple axes or degrees of freedom.

Another object of the invention is to provide a position determiningsystem which provides information on absolute or initial position, ascontrasted with merely relative position based on measurement targetdisplacement.

Another object of the invention is to provide a position determiningsystem which concurrently provides both rotation and translation ofpositional information.

And, another object of the invention is to provide a positiondetermining system which is fast in operation yet provides positionalinformation which is highly accurate.

Briefly, one preferred embodiment of the present invention is ameasuring apparatus. A light source produces light beams for at leasttwo optical channels. In each optical channel, an interferometer isprovided to receive one light beam and provide from it a reference beamand a measurement beam. A reflective target then receives and redirectsthe measurement beam. A beam splitter for receives the redirectedmeasurement beam and provides from it a first and second portions. Adetector for senses the first portion and produces a detector signalbased on it. The interferometer further receives the second portion ofthe measurement beam and combines it with the reference beam to form aresult beam. A receiver is senses the result beam and produces areceiver signal based on it.

An advantage of the present invention is that it provides highlydesirable non-contact position determination, based on its use ofoptical principles. Further, due to its ability to employ lasers as alight source, the invention may be used for measurement targets rangingfrom small to quite large and at distances ranging from near to quiteremote.

Another advantage of the invention is that it permits measurement ofcombinations of positional characteristics which have previously beendifficult to attain with a limited component count. For example, usingtwo parallel optical channels it can measure target translation orstraightness in a perpendicular x-y plane as well as target roll in thex-y plane. The invention is thus effectively able to measure three axesor degrees of freedom with only two sensors. Still additional sensorscan also be added to provide yet further capability

Another advantage of the invention is that its speed of measurement isconsiderably less limited than is the case for relative type measurementsystems. The invention can accurately measure position when a target isat rest, i.e., zero speed, and also when a target is moving quite slowlyor decelerating toward or accelerating from rest. Thus, the presentinvention does not suffer from a “zero-barrier” limitation likeconventional interferometer systems. The invention can also accuratelymeasure position when a target is moving fast, i.e., has a high slewrate. In the present invention, the permissible target speed is limitedmerely by the sensor response times and the electronics used for signalprocessing, which can be quite. This is in marked contrast toconventional laser interferometer systems, which are today severelylimited by the obtainable beam frequency differential.

And, another advantage of the invention is that it is economical toconstruct and operate, particularly in comparison with conventionalsystems producing similar accuracy and measurement speed such as laserinterferometer based systems.

These and other objects and advantages of the present invention willbecome clear to those skilled in the art in view of the description ofthe best presently known mode of carrying out the invention and theindustrial applicability of the preferred embodiment as described hereinand as illustrated in the several figures of the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The purposes and advantages of the present invention will be apparentfrom the following detailed description in conjunction with the appendeddrawings in which:

FIG. 1 (background art) is a perspective view of how a conventionallaser interferometer measurement system is used to determine theposition of a movement stage;

FIG. 2 is an perspective view of an embodiment of the present inventionin use to determine positional information about a movement stage;

FIG. 3 is a block diagram in top plan view particularly showing thedetection section of the embodiment of FIG. 2;

FIG. 4 is a block diagram functional representation of the controlsection;

FIG. 5 is a graph depicting details of the preferred modulation signalprovided by the control section;

FIG. 6 is a side elevation view of the face of the movement stageilluminated in an optimal initial set-up manner by light beams from thedetection section;

FIG. 7 is a side elevation view of the face of the movement stageparticularly showing movement axes, light beam centers, and a light beamseparation distance for the view of FIG. 6;

FIG. 8 is a block diagram in top plan view depicting an alternatepreferred embodiment of the invention;

FIG. 9 a–e depict various side elevation views of the face of themovement stage, in which: FIG. 9 a is of an optimal initial set-up orstarting arrangement, FIG. 9 b is of the movement stage after rightwardmovement, FIG. 9 c is of the movement stage after upward movement, FIG.9 d is of the movement stage after diagonal rightward-upward movement,and FIG. 9 e is of the movement stage after counter-clockwise roll; and

FIG. 10 is a perspective view depicting a sophisticated embodiment ofthe invention forming a multiple parameters measurement system.

BEST MODE FOR CARRYING OUT THE INVENTION

A preferred embodiment of the present invention is a positiondetermining system (hereinafter “PDS”). As illustrated in the variousdrawings herein, and particularly in the views of FIGS. 2 and 5, theinventive device is depicted by the general reference character 100.Where appropriate, reference numbers are reused in the figures.

FIG. 2 depicts a preferred embodiment of the inventive PDS 100 in ageneric usage scenario for determining positional information about atypical movement stage 12, such as that of FIG. 1 (background art).Retroreflective targets 110 are mounted on the movement stage 12; adetection section 112 is provided to optically sense information aboutthe retroreflective targets 110; and a control section 114 is providedto control the operation of the PDS 100 and to communicate with anexternal system 22. The movement stage 12 and the external system 22 arenot formally parts of the inventive PDS 100. As discussed with respectto FIG. 1 (background art) the movement stage 12 may by a simple stageof any type which one wants to know positional information about.Typically, but not necessarily in all applications, the external system22 will control movement of the movement stage 12 using servo feedbacktechniques and also display or record positional information about themovement stage 12 at various points in a manufacturing or laboratoryprocess.

FIG. 3 is a top view particularly showing the components and functionsof the detection section 112. Modulation is applied to one or more lightsources 122, via a bus 120 connected to the control section 114. Detailsfor the preferred form of modulation are discussed presently. Typicallythe light source 122 produces two light beams 124 (two light sources 122are shown here). The inventor's preferred light source 122 includes aconventional laser diode.

Each light beam 124 is passed through a polarizing plate 126. This isdesirable because many light sources, including laser diodes, do notproduce light which is strongly polarized. The polarized light beams 124then pass directly through respective polarized beam splitters 128,which have been suitably positioned and oriented to permit this. Eachlight beam 124 is next passed through a retardation plate 130 (e.g., aconventional ¼ wave plate), where the polarization of the light beam 124is altered. The light beams 124 then exit the detection section 112,proper, and travel onward to and are reflected back by respectiveretroreflective targets 110 which are mounted on the movement stage 12.

The return paths of the light beams 124 are somewhat similar to thosealready taken, but not completely so. After reflection by theretroreflective targets 110, the light beams 124 re-enter the detectionsection 112, and each again passes through its respective retardationplate 130. The already once altered polarizations of the light beams 124are accordingly further altered by this second passage. The light beams124 then re-enter the polarized beam splitters 128. However, due totheir now altered polarizations and the orientations of the polarizedbeam splitters 128, the light beams 124 are each now redirected into arespective sensor unit 132. The sensor units 132 each detect arespective returned light beam 124 and create information which iscommunicated to the control section 114 via the bus 120. The inventor'spreferred sensor unit 132 includes a quad-cell photo detector.

As initially noted, two light beams 124 are typically produced. FIG. 3thus can be viewed as depicting two optical channels 134. The use of twosuch optical channels 134 particularly permits the inventive PDS 100 tomeasure a number of positional characteristics, as will be describedpresently. Embodiments having as few as one and more than two opticalchannels 134 are also possible.

FIG. 4 is a block diagram particularly showing the functional operationof the control section 114. A frequency generator 140 produces amodulation signal 142 which is communicated over the bus 120 to thelight sources 122 in the detection section 112, where it is used tomodulate each light beam 124 (FIG. 4 depicts only one of the opticalchannels 134 of the embodiment of FIG. 3).

FIG. 5 depicts details of the preferred modulation signal 142. Thefrequency of modulation is above 30 kHz, but this is merely a matter ofdesign choice. In applications which the inventor currently has indevelopment, using such a frequency helps to obtain appropriate slaveservo bandwidth in the high speed external system 22. The waveform usedpreferably also has a linear transition 144 at zero crossing. Thisfacilitates electronic signal processing, but also is not a necessity. Asuitable shape for the modulation signal 142 thus might be thetrapezoidal waveform 146 which is shown.

FIG. 6 depicts the faces of both of the sensor units 132 mounted on themovement stage 12 of FIG. 3 (or FIG. 8), being illuminated in their verycenters by the light beams 124. This is an optimal initial set-uparrangement for the PDS 100 and the movement stage 12 since it providesfor ranges of movement in many directions. As shown, when usingquad-cell photo diodes for the sensor units 132, quadrants A, B, C and Dare defined for the left sensor unit 132 and quadrants E, F, G and H aredefined for the right sensor unit 132. The sensor units 132 thus eachhave a center point 202 where the quadrants meet and the respectivelight beams 124 have a center separation 148 (S), as shown. It isdesirable, but not necessary (since compensation in the control section114 can accommodate for some degree of offset), that the center points202 of the sensor units 132 be spaced apart a distance equal to thecenter separation 148 of the light beams 124. Another way of envisioningall of this is to view the light beams 124 as having central axes (notshown in FIG. 6 but easily seen in the stylistically simplified lightbeams 124 of FIG. 3, for example), and appreciating that in an optimumset-up scenario each such central axis intersects the center point 202of a sensor unit 132.

FIG. 7 shows how the movement stage 12 may have an x-axis 204 and ay-axis 206 defined with a common origin 208. For conceptual purposes itis useful to orient the intersections 210 of the quadrants of the sensorunits 132 the same and parallel with the x-axis 204 and y-axis 206, butthe underlying mathematical principles of the invention are not affectedby this.

Returning now to FIG. 4, the sensor units 132 receive the light beams124, in the manner previously described for FIG. 3, and each producesraw signals 150 which are communicated over the bus 120 back to thecontrol section 114. For the quad-cell type sensor units 132 used here,each cell-quadrant produces a raw signal 150, and thus four are createdfor each optical channel 134 (i.e., eight for the embodiment of FIG. 3).

The sensor units 132 are typically direct current (DC) biased, and hencethe raw signals 150 each have both DC and alternating current (AC)signal elements at this early stage. Unfortunately, the DC element issubject to thermal and other types of drift, which is a particularweakness of many present detectors. The effects of such drift must beeliminated before high gain amplification is used in later signalprocessing. To remove the undesirable DC elements, the raw signals 150are fed into differential amplifiers 152 which couple only the ACelements and create quadrant A–B, B–C, C–D, and D–A difference signals154.

Each difference signal 154 and the respective components used to furtherprocess it may collectively be viewed as an electrical channel 156. FIG.4 depicts only one such electrical channel 156, but the embodiment ofFIG. 2 might employ eight such channels or use multiplexing (not shown)to lower the electrical channel count. The inventive spirit of the PDS100 encompasses such alternates.

Once the difference signals 154 have the undesirable DC elements removedthere are usually still undesirable AC elements also present. Forexample, room lighting may introduce such undesirable AC elements. Ifincandescent lighting is used in a measurement area it may introduce 60hertz AC signal elements, and if fluorescent lighting is used it mayintroduce various higher frequency AC signal elements. Eliminating suchundesirable AC elements is of key importance, and the manner in whichthe inventor does this is new to the art of optical positionmeasurement.

A sample of each difference signal 154 is processed by a firstsynchronous demodulator 158 and passed through a first low pass filter160 to obtain a coarse position signal 162. Since the first synchronousdemodulator 158 operates directly on the low gain difference signal 154,high precision demodulation is not required here, and conventionalanalog switches and operational amplifiers may be used.

Another sample from each difference signal 154 is amplified with a highgain amplifier 164 to produce a highly amplified signal 166. In thepreferred embodiment, an operational amplifier configured as aninverting amplifier is used for the high gain amplifier 164, to providea gain of 500 and to thereby obtain heightened sensitivity in the PDS100. The highly amplified signal 166 is then processed by a secondsynchronous demodulator 168, and is passed through a second low passfilter 170 to obtain a fine position signal 172. The second synchronousdemodulator 168 usually must be of high precision, due to the sensitivenature of the highly amplified signal 166.

In the preferred embodiment the second synchronous demodulator 168 andthe second low pass filter 170 are combined in a board level, lock-inamplifier system which serves as a high quality balanced demodulator anda 6th order high quality filter. A suitable component for this is a“Lock-in Engine” which is commercially available from Quanta Physik,Inc. of Palm Beach Gardens, Fla., USA. (“The lock-in amplifier isbasically a synchronous demodulator followed by a low pass filter . . .Lock-in amplification is a technique which is used to separate small,narrow band signal content from interfering noise. The lock-in amplifieracts as a detector and narrow band filter combined. Very small signalscan be detected in the presence of large amounts of non-correlated noisewhen the frequency and phase of the desired signals are known.” FromAD630 Application Note by Analog Devices, Inc. of Norwood, Mass. QuantaPhysik's Lock-in Engine is built around the AD630 component.)

The coarse position signal 162 and the fine position signal 172 areprovided to a logic unit 174, and optionally also directly to theexternal system 22. The logic unit 174 will typically include powerfulmicroprocessor capabilities which will depend considerably on thecapabilities of the external system 22 and the needs of the applicationin which the PDS 100 is used.

A communications link 176 is provided between the logic unit 174 and theexternal system 22. This communications link 176 may be bi-directional,permitting the external system 22 to transmit instruction signals 178 tothe logic unit 174 for when to operate the PDS 100 and obtain the coarseposition signal 162 and fine position signal 172, and also permittingthe PDS 100 to transmit processed position data in a result signal 180back to the external system 22.

FIG. 8 depicts an alternate preferred embodiment of the inventive PDS100. Here, the sensor units 132 are instead directly mounted on themovement stage 12, in place of the retroreflective targets 110 of FIG.2, and a targeting section 190 optically “targets” the remote sensorunits 132. The same control section 114 as previously described can alsocontrol the operation of the PDS 100 and communicate with the externalsystem 22 in this embodiment.

The targeting section 190 used here may be much simpler optically thanthe detection section 112 of FIGS. 2 and 3. A light source 122 again ispresent and used to provide light beams 124 which are modulated with themodulation signal 142 from the control section 114. A major change,however, is that no polarization related components are needed. Thesensor units 132 which are mounted directly on the movement stage 12 areilluminated directly by the light beams 124, without using reflectionand any intervening optical components between them and the lightsources 122. The sensor units 132 may work essentially the same aspreviously discussed for FIG. 2–4, producing the same raw signals 150.

While optically much simpler, this alternate embodiment may sometimes bemore complex mechanically and electronically, and accordingly moretroublesome to employ. For example, if the separation between the lightsources 122 and the sensor units 132 is great, the electrical cable usedto carry the raw signals 150 may be easily abused and damaged. Also, theraw signals 150 may be unduly attenuated or corrupted by electricalnoise when traveling long distances.

FIG. 9 a–e depict side elevation views of the face of the movement stage12 of FIG. 8. Further, as those skilled in the optical arts willappreciate, the conceptual principle in the following discussion appliesto the faces of the sensor units 132 in the embodiment of FIG. 2–3 aswell. The faces of the sensor units 132 are depicted here as havingquadrants A–D and E–H illuminated by the light beams 124.

FIG. 9 a is essentially the same as FIG. 7. It shows an optimal initialplacement of the movement stage 12 relative to the detection section 112or the targeting section 190. The faces of both sensor units 132 areilluminated in their very centers by the light beams 124. Quadrants A,B, C and D on the left sensor unit 132 and quadrants E, F, G and H onthe right sensor unit 132 are all receiving equal illumination, and theraw signals 150 going to the control section 114 will indicate this. Therespective light beams 124 have a center separation 148 (S), as shown.

FIG. 9 b shows the movement stage 12 displaced laterally to the left,i.e. horizontally, from where it was in FIG. 9 a. The direction ofmovement is thus perpendicular to the propagation direction of the lightbeams 124. The illumination on the quadrants here has changed, and theraw signals 150 going to the control section 114 will now indicate this.Suitable processing in the control section 114 will therefore be able todetermine if and to what extent such movement has occurred. This permitsthe PDS 100 to perform horizontal straightness measurement.

FIG. 9 c shows the movement stage 12 displaced laterally upward, i.e.vertically, from where it was in FIG. 9 a. The direction of movement isagain perpendicular, but differently so, with respect to the light beams124. The illumination on the quadrants has also changed here, as the rawsignals 150 will again indicate. Suitable processing in the controlsection 114 is also able to determine if and to what extent thismovement has occurred. This permits the PDS 100 to perform verticalstraightness measurement.

FIG. 9 d is a more complex case. It shows the movement stage 12displaced diagonally upward and to the left, i.e. both horizontally andvertically, from where it was in FIG. 9 a. However, this is also wellhandled by the inventive PDS 100.

FIG. 9 e is a still more complex case. It shows the movement stage 12rotated about an axis 182 (which is perpendicular to the plane of thefigure and therefore depicted accordingly). In discussing FIG. 1(background art), this type of tilt or rotational movement wasidentified as “roll.” The PDS 100 can perform roll measurement by usingappropriate processing in the control section 114 of the raw signals 150which occur here.

The underlying principles of how the inventive PDS 100 can perform theabove and other forms of translation and roll measurement are asfollows. When the light beams 124 illuminate the sensor units 132 theyproduce a current or voltage (depending on the type of sensor used)which is proportional to the strength of the light present. As wasdiscussed above, the raw signals 150 are processed to eliminateundesirable elements not attributable to the light beams 124, e.g.,drift, interference from room lighting, etc.

In the preferred embodiment, the photo detector components used in thesensor units 132 each produces a current (I) for each quadrant which isproportional to the illumination received. Thus, the illumination fromthe left light beam 124 on the left sensor unit 132 is defined by theequation I₁=I_(A)+I_(B)+I_(C)+I_(D). Similarly, the illumination fromthe right light beam 124 on the right sensor unit 132 is defined by theequation I_(r)=I_(E)+I_(F)+I_(G)+I_(H).

The movement of the movement stage 12 depicted in FIG. 9 b and 9 c,i.e., strictly horizontal or vertical translation, may be found usingthe simple equations:ΔX=I _(A) −I _(B) =I _(D) −I _(C) =I _(E) −I _(F) =I _(H) −I _(G);ΔY=I _(A) −I _(D) =I _(B) −I _(C) =I _(E) −I _(H) =I _(F) −I _(G).From these it can be seen that if one is only concerned about a strictlyhorizontal or a strictly vertical translation, one can even dispensewith using quad-cell components and simply use appropriately orientedbi-cells in the sensor units 132.

For horizontal and vertical translation in combination, such as thesituation depicted in FIG. 9 d, the equations above will not work. Forstrictly horizontal and vertical translation together one may insteaduse:ΔX=(I _(A) +I _(D))−(I _(B) +I _(C))=(I _(E) +I _(H))−(I _(F) +I _(G));ΔY=(I _(A) +I _(B))−(I _(C) +I _(D))=(I _(E) +I _(F))−(I _(G) +I _(H)).

However, even these equations are not accurate if roll occurs. Forignoring roll and detecting just the horizontal and verticaltranslations one may use:ΔX=the lesser of either (I _(A) +I _(D))−(I _(B) +I _(C)) or (I _(E) +I_(H))−(I _(F) +I _(G));ΔY=the lesser of either (I _(A) +I _(B))−(I _(C) +I _(D)) or (I _(E) +I_(F))−(I _(G) +I _(H)).And for determining the amount of roll (lets call this “θ”) of themovement stage 12 one may use the equation (recalling that S=the centerseparation 148 of the light beams 124):Δθ=(((I _(A) +I _(D))−(I _(B) +I _(C)))−((I _(E) +I _(H))−(I _(F) +I_(G))))/S.

Returning now primarily to FIG. 3, several changes can be made in theembodiment depicted there without departing from the spirit of thepresent invention. For example, the multiple light sources 122 andpolarizing plates 126 might be replaced with single instances of each,and beam splitting and bending instead used to produce the desirednumber of light beams 124. The inventor prefers the depicted versionbecause laser diodes and polarizing plates are relatively inexpensiveand easy to work with. In contrast, beam splitters and benders havingthe requisite quality, such as partially reflective cubes and mirrors,are expensive and add to optical component alignment difficulties.

Another example would be to exchange the positions of the light sources122 and polarizing plates 126 with those of the sensor units 132, and tosuitably orient the polarized beam splitters 128 for working with thisarrangement instead. Under this variation, the light beams 124 wouldinitially be reflected by the polarized beam splitters 128, and thenlater pass directly through them when returning from the retroreflectivetargets 110.

Returning to FIG. 2–3 and FIG. 8, the embodiment of FIG. 2–3 isparticularly suitable for movement stages which are distant from thedetection section 112, or which move at high enough speeds that routingthe bus 120 to it is undesirable. In contrast, the embodiment of FIG. 8is simpler and cheaper. But, yet other embodiments are also possible,and are encompassed within the spirit of the inventive PDS 100. Forexample, a hybrid approach would be to angularly reflect the light beamsoff of reflectors mounted on the movement stage and onto sensor unitsmounted elsewhere off of the movement stage. This would provide theoptical economy of the FIG. 8 embodiment and the tether-less cableadvantage of the FIG. 2–3 embodiment, but at the expense of aligning andmaintaining the angular light beam reflection paths.

FIG. 10 is a perspective view depicting a more sophisticated embodimentof the inventive PDS 100, one which the inventor terms a “multipleparameters measurement system” (MPMS 300). The MPMS 300 includes asingle laser light source 302 to provide a light beam 304, that is splitby a splitter 306 into two parts, one of which is then directed parallelwith the other by a beam bender 308. This permits the one light source302 to serve two optical channels. Each channel includes a linearinterferometer 310, a beam splitter 312, retroreflector 314 (mounted ona measurement target, not shown), a detector 316, and a receiver 318.Polarization may or may not be used, in the manner already described forthe embodiment in FIG. 3. The detectors 316 are “position sensitivedetectors,” and may be photodiode devices such as those described forthe sensor units 132 (FIGS. 3 and 8)(e.g., bi-cell or quad-cell units,diode arrays etc.). In contrast, while photodiode devices may also beused in the receivers 318 the function there is to detect a beatfrequency rather than detection. Accordingly, these preferably areprecision single cell units.

Operationally, in each channel, the light beam 304 enters the linearinterferometer 310, where it is split into a reference beam and ameasurement beam 320. The measurement beam 320 then passes through thebeam splitter 312 and travels to and is reflected by the retroreflector314 back to the beam splitter 312. This time however, the measurementbeam 320 is split at the beam splitter 312 into a first beam portionwhich travels to the detector 316 and a second beam portion which entersthe linear interferometer 310. In the linear interferometer 310 thissecond beam portion and the reference beam combine, interference occursin the characteristic manner when waves combine, and the resulting beamtravels on to the receiver 318 as shown.

When the retroreflector 314 undergoes lateral movement (due torotational or translational change) the illumination of the first beamportion of the measurement beam 320 on the detectors 316 changes, andthis is measurable with the respective detector 316.

When the retroreflector 314 undergoes displacing movement (due to lineardisplacement or yaw) a Doppler shift occurs which is proportional to thespeed of this movement. The frequency of the reference beam, however,has remained constant while the frequency of the second beam, portion ofthe measurement beam 320 has changed. The interference accordinglychanges, and the receiver 318 senses this change. With suitablefrequency counting and accumulation in a control section (not shownhere; see e.g., FIG. 2) the longitudinal displacement of theretroreflector 314 can thus be determined.

Of course, this occurs in both optical channels concurrently. Therefore,linear displacement can be calculated as the longitudinal displacementof one retroreflector 314 plus the longitudinal displacement of theother retroreflector 314, divided by two. Yaw can be calculated as thelongitudinal displacement of one retroreflector 314 minus thelongitudinal displacement of the other retroreflector 314, divided bytwo. Horizontal straightness movement can be calculated as thehorizontal position reading of one detector 316 plus the horizontalposition reading of the other detector 316, divided by two. Verticalstraightness movement can be calculated as the vertical position readingof one detector 316 plus the vertical position reading of the otherdetector 316, divided by two. Roll can be calculated as the arctangentof the difference in the vertical position readings divided by thedifference in the horizontal position readings.

In sum, the MPMS 300 can measure movement of an object with five degreesof freedom. Only measuring pitch proves difficult, but instraightforward manner one or more optical channels can be added and itcan be measured as well.

The MPMS 300 can, optionally, use modulation of the light beam 304 andphase sensitive detection, if accuracy necessitates that. However, inmany cases that can be dispensed with, since performing 5-degreemeasurement with a single “station” comprising one MPMS 300 is useful inmany measurement scenarios in its own right.

In addition to the above mentioned examples, various other modificationsand alterations of the inventive PDS 100 may be made without departingfrom the invention. Accordingly, the above disclosure is not to beconsidered as limiting and the appended claims are to be interpreted asencompassing the true spirit and the entire scope of the invention.

INDUSTRIAL APPLICABILITY

The present position determining system (“PDS 100”) and the multipleparameters measurement system (MPMS 300) are well suited for applicationin detecting positional characteristics of fixed and moving measurementtargets. In many industrial processes measurement stages 12 are suitableas such measurement targets, and thus these industrial process maybenefit by use of the inventive PDS 100 or MPMS 300. Example industrieswhere particular present need exists include semiconductor devicefabrication and disk storage unit assembly. The invention providesmeasurement accuracy and speed which are desired in these industries, aswell as many others.

The PDS 100 and MPMS 300 may also be highly desirable for some types ofmeasurement due to the non-contact nature of its optical principles. Useof the invention can thus avoid undesirable interference with anunderlying manufacturing process. Suitable embodiments of the PDS 100and MPMS 300 may also overcome the range limitations of some commonelectrical measurement systems. Measurement systems using transducerssuch as LVDT and capacitive sensors have notoriously short measurementranges, and considerable loss of reliability as the extremes of theirranges are approached. Whereas such electrical systems are typicallyuseful only at ranges of less than one meter, optical systems, includingthe PDS 100, may be used at ranges up to many kilometers.

The PDS 100 and MPMS 300 are also economical. It is relativelyinexpensive to construct and it is often cheaper to operate than manyexisting measurement systems. This is particularly notable in contrastto particular present measurement systems used in roles which theinventive PDS 100 and MPMS 300 may now fill. For example, laserinterferometer systems are notoriously expensive, and the unreliabilityof electrical systems, alluded to above, can be quite uneconomical ifmanufacturing tool breakage or material scrapage occurs as a result.

As has already been described here, the PDS 100 and MPMS 300 may beconstructed of relatively common and available electro-opticalcomponents such as laser diodes and photo diodes; optical componentssuch as polarizers, beamsplitters, quarter-wave plates, andretroreflectors; and electronic components such as amplifiers, filters,and microprocessors. In view of these and other characteristics,successful and rapid construction of various embodiments of theinventive PDS 100 and MPMS 300 should be well within the capabilities ofskilled practitioners of the relevant arts once the principles taughtherein are appreciated.

For the above and other reasons, it is expected that the PDS 100 andMPMS 300 of the present invention will have widespread industrialapplicability and it is expected that the commercial utility of thepresent invention will be extensive and long lasting.

1. A measuring apparatus, comprising: a light source for producing lightbeams for at least two optical channels, wherein said light sourceincludes a modulator to produce said light beams including a modulationcharacteristic; said optical channels each including: an interferometerfor receiving one said light beam and providing therefrom a referencebeam and a measurement beam; a reflective target for receiving andredirecting said measurement beam; a beam splitter for receiving theredirected said measurement beam and providing therefrom a first portionand a second portion; a detector for sensing said first portion andproducing a detector signal based thereon; said interferometer furtherfor receiving said second portion of said measurement beam and combiningsaid second portion with said reference beam to form a result beam; anda receiver for sensing said result beam and producing a receiver signalbased thereon; and a processing system for processing said detectorsignals and said receiver signals into position data suitable forcommunication to an external system, wherein said processing systemincludes a demodulator to process at least one of said detector signalsand said receiver signals with phase sensitive detection.
 2. Themeasuring apparatus of claim 1, wherein said light source includes alaser diode.
 3. The measuring apparatus of claim 1, wherein said lightsource includes a single light producing unit, a splitter and a benderfor producing said light beams.
 4. The measuring apparatus of claim 1,wherein said light source includes a plurality of light producing units,one per each said optical channel.
 5. The measuring apparatus of claim1, wherein said interferometers and said beam splitters employpolarization.
 6. The measuring apparatus of claim 1, wherein saidreflective targets are retroreflectors.
 7. The measuring apparatus ofclaim 1, wherein said detectors are position sensitive detectors.
 8. Themeasuring apparatus of claim 1, wherein said detectors include at leastone member of the set consisting of bi-cell photo diode units, quad-cellphoto diode units, and photo diode arrays.
 9. The measuring apparatus ofclaim 1, wherein said receivers include photo diodes.
 10. A measuringapparatus, comprising: means for producing light beams for at least twooptical channels, wherein said means for producing light beams includesmeans for modulating to produce said light beams including a modulationcharacteristic; said optical channels each including: interferometermeans for receiving one said light beam and providing therefrom areference beam and a measurement beam; means for receiving andredirecting said measurement beam; splitter means for receiving theredirected said measurement beam and providing therefrom a first portionand a second portion; detector means for sensing said first portion andproducing a detector signal based thereon; said interferometer meansfurther for receiving said second portion of said measurement beam andcombining said second portion with said reference beam to form a resultbeam; and receiver means for sensing said result beam and producing areceiver signal based thereon; and processing means for processing saiddetector signals and said receiver signals into position data suitablefor communication to an external system, wherein said processing meansincludes demodulating means to permit processing at least one of saiddetector signals and said receiver signals with phase sensitivedetection.
 11. The measuring apparatus of claim 10, wherein: said meansfor producing light beams includes: means for producing an initial beam;means for splitting said initial beam into a first beam and at least onesecondary beam; and bender means for directing said secondary beams inparallel with said first beam, thereby producing said light beams forsaid at least two optical channels.
 12. The measuring apparatus of claim10, wherein: said interferometers include means for polarizing saidmeasurement beams; and said splitter means includes means for separatingwith polarization, thereby permitting providing said first portions andsaid second portions of said measurement beams based on respectivepolarization characteristics.
 13. A method for measuring positionalinformation about a target, the method comprising the steps of: (a)producing light beams for at least two optical channels, includingmodulating said light beams with a frequency; and in each said opticalchannel: (b) receiving a said light beam and providing therefrom areference beam and a measurement beam; (c) receiving at and redirectingsaid measurement beam from the target; (d) receiving the redirected saidmeasurement beam and providing therefrom a first portion and a secondportion; (e) producing a detector signal based on said first portion;(f) combining said second portion with said reference beam to form aresult beam; (g) producing a receiver signal based on said result beam;and (h) processing said detector signals and said receiver signals intoposition data suitable for communication to an external system,including demodulating at least one of said detector signals and saidreceiver signals based on said frequency and applying phase sensitivedetection.
 14. The method of claim 13, wherein: said step (b) includespolarizing said measurement beams; and said step (d) includes separatingsaid first portions from said second portions based on polarization.